Improvements relating to carbon fibre processing

ABSTRACT

Carbon fibre precursors for use in the formation of carbon fibre materials. The carbon fibre precursors comprise fibres of polymeric material which have a coating layer thereon, the coating layer comprising a material susceptible to dielectric heating, for example carbon nanotubes. The carbon fibre precursors may be suitable for forming into carbon fibres using a dielectric heating step, despite the fibres of polymeric material not being susceptible to dielectric heating, without adversely affecting the structure and physical properties of the main body of the carbon fibre so formed. A method of preparing a carbon fibre precursor for a carbon fibre formation process and a method forming a carbon fibre are also disclosed.

FIELD

The present invention relates to a carbon fibre precursor, a method ofpreparing a carbon fibre precursor for a carbon fibre formation processand a method of forming a carbon fibre. In particular the inventionrelates to carbon fibre precursors which can be carbonized usingdielectric heating.

BACKGROUND

Carbon fibres are commonly used for many structural applications such asin the aerospace, military, automobile and wind turbine industries.Currently, the vast majority of carbon fibres are produced by heattreatment and pyrolysis (carbonization) of polyacrylonitrile (PAN)carbon fibre precursors which are synthesised from petroleum sources.There are several disadvantages with this use of PAN as carbon fibreprecursors, for example high cost, slow carbonization and thedetrimental environmental impact of the acrylonitrile productionprocess.

Lignin may provide a more environmentally benign alternative for carbonfibre production. Lignin is a complex organic polymer present in thecell walls of pith, roots, fruit, buds and bark and, along withhemicellulose and cellulose, is one of the most abundant components oflignocellulosic biomass. However, lignin itself performs poorly duringthe typical melt spinning process used to form carbon fibre precursors,which makes industrial scale production extremely complicated anddifficult. Lignin may also provide carbon fibre precursors of arelatively poor quality, for example such lignin-derived carbon fibreprecursors may comprise voids which may adversely affect the physicalproperties of the carbon fibres produced from such precursors.

A key step in the production of carbon fibres is carbonization. In thecarbonization step, carbon fibre precursors are heated to temperaturesof 800-3,000° C. (depending on the type of carbon fibre precursor) inthe absence of oxygen to expel non-carbon atoms from the carbon fibreprecursors. This produces carbon fibre yarns comprising mainly carbonatoms and very few non-carbon atoms. The carbon fibre yarns can then befurther processed to facilitate incorporation into products, often ascomposites with polymeric materials.

The carbonization step in carbon fibre production is particularly energyintensive due to the high temperatures involved. Currently, carbonfibres are carbonized using traditional heating system such as ovens andfurnaces which have relatively high energy consumption, which in turnimpacts on the environmental profile and the production costs of carbonfibres.

It would therefore be desirable to reduce the energy consumption ofcarbon fibre production processes, particularly the carbonization step,to reduce the environmental impact of carbon fibres. In particular itmay be desirable to make such a reduction in energy consumption in theprocessing of lignin-based carbon fibre precursors, which as mentionedabove are themselves less environmentally impactful than other carbonfibre precursors such as PAN, in order to further reduce theenvironmental impact of carbon fibre production.

SUMMARY OF THE INVENTION

It is one aim of the present invention, amongst others, to provide acarbon fibre precursor, a method of preparing a carbon fibre precursorfor a carbon fibre formation process and a method of forming a carbonfibre that addresses at least one disadvantage of the prior art, whetheridentified here or elsewhere, or to provide an alternative to existingmethods. For instance it may be an aim of the present invention toprovide a carbon fibre precursor which can be carbonized into a carbonfibre using less energy than current carbon fibre precursors.

According to aspects of the present invention, there is provided acarbon fibre precursor and method as set forth in the appended claims.Other features of the invention will be apparent from the dependentclaims, and the description which follows.

According to a first aspect of the present invention, there is provideda carbon fibre precursor comprising a fibre of polymeric material and acoating layer on the fibre, the coating layer comprising a dielectricheating susceptor material.

The inventors have found that by incorporating a coating layercontaining a dielectric heating susceptor material, the carbon fibreprecursors of this first aspect can be carbonized using dielectricheating, such as microwave (MW) and radio frequency (RF) heating whichmay use less energy than known methods of carbonizing known carbon fibreprecursors. The susceptibility of lignin and PAN to dielectric heatingis very low and therefore carbonization of such carbon fibre precursorsusing dielectric heating was found to be ineffective. Providing thecoating comprising a dielectric heating susceptor material allows suchcarbon fibre precursors, for example lignin- and PAN-based carbon fibreprecursors, to be effectively carbonized using dielectric heating,without adversely affecting the structure and physical properties of themain body of the carbon fibre (produced from the fibre of polymericmaterial after carbonization).

The fibre of polymeric material on which the coating is provided may beany suitable carbon fibre precursor fibre material. This fibre isintended to be converted to a carbon fibre during carbonization and anyother process step required. The dielectric heating susceptor material,which may provide the advantageous dielectric heating carbonization ofthe carbon fibre precursor, is located in the coating layer and suitablynot in the fibre of polymeric material. The fibre of polymeric materialis suitably substantially free of the dielectric heating susceptormaterial or any other dielectric heating susceptor material. The fibreof polymeric material is suitably unresponsive to dielectric heating,suitably having a dielectric constant of less than 20, suitably lessthan 10 or less than 5.0.

The inventors have found that providing the dielectric heating susceptormaterial in the fibre of polymeric material may have the drawbacks ofinhomogeneous distribution of the susceptor material and a larger amountof susceptor material being required to have the desired effect. Perhapsmost importantly, providing the dielectric heating susceptor material inthe fibre of polymeric material may cause defects to form in the carbonfibre produced from the carbon fibre precursor during carbonization,which adversely affects the mechanical properties of said carbon fibre.Therefore the carbon fibre precursors of this first aspect mayadvantageously use a lower amount of susceptor material than wouldotherwise be required and may also avoid the structural defects producedby susceptor materials in the fibre of polymeric material, whilstenabling carbonization by dielectric heating with the reduced energyconsumption described above.

The fibre of polymeric material may be formed from a synthetic polymeror a biologically derived polymer, or a mixture thereof. Suitablesynthetic polymers may be ultimately derived from mineral oil. Asuitable synthetic polymer may selected from any one or more ofpolyacrylonitrile (PAN), nylon, polyethylene terephthalate or apolyester. Suitably the fibre of polymeric material is a PAN fibre. PANis not sufficiently susceptible to dielectric heating for dielectricheating to be used for carbonizing PAN fibres, without adding dielectricheating susceptor materials.

Suitable biologically derived or “natural” polymers include lignin.Suitably the fibre of polymeric material comprises lignin. It isbelieved that any type of lignin can be utilised in the fibre ofpolymeric material, for example lignin obtained from softwood, hardwoodor grass/annual plants. Suitable lignin can be obtained from thesesources using various known processes, for example the Kraft,organosolve or soda processes. In some embodiments, more than one typeand/or source of lignin is used to provide the lignin of the fibre ofpolymeric material. Lignin is not sufficiently susceptible to dielectricheating for it to be used for carbonizing lignin fibres, without addingdielectric heating susceptor materials.

Suitably the fibre of polymeric material comprises a lignin and at least10 wt % of a thermoplastic elastomer. Suitably the fibre of polymericmaterial consists essentially of the lignin and the thermoplasticelastomer. Suitably the fibre of polymeric material consists of thelignin and the thermoplastic elastomer.

The thermoplastic elastomer may be a mixture of thermoplastic elastomermaterials or may be a single thermoplastic elastomer material. Suitablythe thermoplastic elastomer is a polymeric material. Suitably the fibreof polymeric material does not comprise any other polymeric materialsexcept for the lignin and the thermoplastic elastomer.

Suitably the lignin and the thermoplastic elastomer are thoroughly mixedin the fibre of polymeric material. The fibre of polymeric material maybe considered to be a blend of lignin and thermoplastic elastomer.

The inventors have found that the combination of lignin andthermoplastic elastomer provides a composition which has improvedproperties for processing into a carbon fibre precursor, compared toknown compositions comprising only lignin. For example, the compositionof lignin and thermoplastic elastomer may be extruded effectively into afibre which may then be wound onto a bobbin without breaking. It isbelieved that the thermoplastic elastomer advantageously modifies themechanical properties of the lignin to increase the normally lowtenacity and flexibility of the lignin sufficiently to allow processinginto carbon fibre precursors and subsequently into carbon fibres.

Suitably, at least a part of the thermoplastic elastomer comprisesfunctional groups which provide compatibility with lignin. Compatibilitywith lignin may be determined by the polarity of the polymer and/orfunctional groups within the polymer. Semi-polar polymers may provideacceptable compatibility with lignin. For example, polyester polyols andpolyether polyols may have an appropriate polarity for compatibilitywith lignin. Said semi-polar polymers may provide parts or segments ofthe thermoplastic elastomer. Said semi-polar polymers, for examplepolyester polyols or polyether polyols may provide compatibility withlignin and enable the thermoplastic elastomer to combine with the ligninto provide a fibre of polymeric material with the improved mechanicalproperties discussed herein.

The inventors have found certain polyurethanes to be particularlyadvantageous in providing compatibility with lignin and in providingdesirable mechanical properties in the fibre of polymeric material, forexample polyurethanes comprising (i.e. formed from) polyether polyols orpolyester polyols. Thermoplastic polyurethanes formed from adiisocyanate comprising an aryl group and polyether polyols or polyesterpolyols may be particularly advantageous.

An example of a suitable thermoplastic polyurethane is TPU PearlthaneECO 12T95 supplied by Lubrizol.

Pearlthane ECO 12T95 is formed from a polyester diol derived from castoroil (to provide soft segments), MDI, dipropylene glycol (minor amount)and 1,4-butanediol (as a “chain extender” providing hard segments onreaction with the MDI).

The fibre of polymeric material may comprise at least 10 wt % of thethermoplastic elastomer and/or mixtures of thermoplastic elastomers.Suitably the fibre of polymeric material comprises at least 20 wt % ofthe thermoplastic elastomer, suitably at least 25 wt %, suitably atleast 30 wt %, suitably at least 35 wt %, suitably at least 40 wt %.

Suitably the fibre of polymeric material comprises up to 60 wt % of thethermoplastic elastomer, suitably up to 55 wt %, suitably up to 50 wt %.

Suitably the fibre of polymeric material comprises from 10 to 60 wt % ofthe thermoplastic elastomer, suitably from 20 to 60 wt %, suitably from25 to 55 wt % or from 25 to 50 wt %. Suitably the fibre of polymericmaterial comprises a crosslinking agent. Said crosslinking agent mayprovide crosslinks between chains of the lignin or between the chains ofthermoplastic elastomer on formation of fibre of polymeric material, forexample by reactive extrusion.

The crosslinking agent may be selected from any one or more ofisocyanates (for example 4,4′-methylene diphenyl isocyanate) anddiglycidyl compounds (for example neopentyl glycol diglycidyl ether).Other suitable chemical species capable of reacting with hydroxyl groupsto enable the crosslinking of polymer chains may also be used as saidcrosslinking agent.

Suitably the fibre of polymeric material is a PAN fibre or a fibrecomprising lignin.

The fibre of polymeric material suitably has a thickness of from 1 to100 μm.

The carbon fibre precursor of this first aspect comprises a coatinglayer on the fibre which comprises a dielectric heating susceptormaterial. The coating layer is suitably a substantially uniform coatingaround the circumference and along the length of the fibre of polymericmaterial of the carbon fibre precursor. Suitably the coating layer has athickness of from 5 to 200 nm, suitably from 10 to 150 nm, suitably from20 to 125 nm or from 25 to 100 nm. The inventors have found that thisthickness of a coating comprising a dielectric heating susceptormaterial is sufficient to heat the fibre of polymeric material during adielectric heating process to a temperature which carbonizes the fibreto form a carbon fibre. During said dielectric heating process, thecoating composition becomes part of the carbon fibre.

Suitably the coating later comprises a polymeric carrier material.Suitably the polymeric carrier material is an ionic polymer. A suitableionic polymer may be selected from poly(diallyldimethylammoniumchloride) (PDDA), poly(styrenesulfonate) (PSS), polyacrylic acid (PAA),poly(allylamine hydrochloride), a carboxymethyl cellulose, an alginateor mixtures thereof.

Suitably the coating layer comprises a surfactant. Suitably thesurfactant is compatible with and/or can interact with the polymericcarrier material and the dielectric heating susceptor material, in orderto stabilise the dielectric heating susceptor material in the coating onthe fibre of polymeric material. Suitably the surfactant is an ionicsurfactant. The surfactant may be selected from sodium deoxycholate(DOC), cetrimonium bromide (CTAB), sodium dodecyl sulfate (SDS) orsodium dodecylbenzenesulfonate (SDBS), or mixtures thereof.

Suitably the dielectric heating susceptor material is present in thecoating layer in a greater amount than the polymeric carrier materialand the surfactant (when present).

Suitably the polymeric carrier material and the surfactant are bothionic. Suitably the polymeric carrier material is cationic and thesurfactant is anionic, or the polymeric carrier material is anionic andthe surfactant is cationic. This may provide an ionic interactionbetween the polymeric carrier material and the surfactant whichstabilises the coating layer comprising the dielectric heating susceptormaterial on the fibre of polymeric material.

The coating layer comprises the dielectric heating susceptor material.By dielectric heating susceptor material we mean to refer to a material,for example a particulate material, which absorbs electromagneticradiation and converts said electromagnetic radiation to heat. Forexample, the dielectric heating susceptor material may absorb radiofrequency radiation and/or microwave radiation and convert saidradiation to heat. Suitably the dielectric heating susceptor materialabsorbs electromagnetic radiation and converts said electromagneticradiation to heat to a greater extent than the fibre of polymericmaterial, suitably to a much greater extent. Suitably the dielectricheating susceptor material absorbs electromagnetic radiation andconverts said electromagnetic radiation to heat to a sufficient extentto heat and carbonize the carbon fibre precursor of this first aspect toproduce carbon fibres.

The dielectric heating susceptor material is suitably selected from anyone or more of hollow nanospheres, nanotubes, nanofibres, nanosheets,graphene, graphene derivatives and nano/micro hybrids. The dielectricheating susceptor material may also be nanorods, suitably carbonnanorods. These materials may be alternatively or additionally definedas low dimensional particles, for example particles with at least onenanoscale dimension or component.

Suitably the dielectric heating susceptor material is nanoscaleparticles. Suitably the dielectric heating susceptor material has aparticle size in the range of 50 nm to 1,000 nm (measured bytransmission electron microscopy (TEM) using standard techniques).

Suitably the dielectric heating susceptor material is formed of carbonnanotubes.

In the context of the present invention, the term “carbon nanotube”refers to a structure conceptually similar to that made by rolling up asheet of graphene into a cylinder. Depending on the rolling degree andthe way the original graphene sheet is formed, carbon nanotubes ofdifferent diameter and internal geometry can be formed. Carbon nanotubesformed by rolling up of a single sheet forming the aforementionedcylinder, are called “single-walled” carbon nanotubes (SWCNTs). Thecarbon nanotubes formed by rolling up more than one sheet of graphenewith a structure that resembles a series of concentric cylinders ofincreasing diameters from the center to the periphery are called“multi-walled” carbon nanotubes (MWCNTs).

Suitably the dielectric heating susceptor material is formed ofmulti-walled carbon nanotubes (MWCNTs).

In embodiments wherein the carbon nanotubes are multi-walled carbonnanotubes, the multi-walled carbon nanotubes suitably comprise from 2 to5 graphitic layers.

The carbon nanotubes suitably have a high aspect ratio(length-to-diameter ratio), suitably an aspect ratio of between 10 and10,000,000 to 1, suitably between 100 and 10,000,000 to 1. The carbonnanotubes are also suitably highly graphitic.

Suitably the dielectric heating susceptor material provides from 0.01 to0.1 wt % of the carbon fibre precursor.

Suitably the carbon fibre precursor of this first aspect comprises afibre of polymeric material and a coating layer on the fibre, thecoating layer comprising carbon nanotubes (as a dielectric heatingsusceptor material) and a surfactant, the coating layer having athickness of from 10 to 150 nm.

Suitably the carbon fibre precursor of this first aspect comprises afibre of PAN and a coating layer on the fibre, the coating layercomprising carbon nanotubes (as a dielectric heating susceptor material)and a surfactant, the coating layer having a thickness of from 10 to 150nm.

Suitably the carbon fibre precursor of this first aspect comprises afibre of a blend of lignin and a thermoplastic elastomer (for exampleTPU) and a coating layer on the fibre, the coating layer comprisingcarbon nanotubes (as a dielectric heating susceptor material) and asurfactant, the coating layer having a thickness of from 10 to 150 nm.

Suitably the carbon fibre precursor of this first aspect comprises afibre of polymeric material and a coating layer on the fibre, thecoating layer comprising carbon nanotubes (as a dielectric heatingsusceptor material), an ionic surfactant and an ionic polymeric carriermaterial, the coating layer having a thickness of from 10 to 150 nm.

Suitably the carbon fibre precursor of this first aspect consistsessentially of a fibre of polymeric material and a coating layer on thefibre, the coating layer consisting essentially of a dielectric heatingsusceptor material, a surfactant and a polymeric carrier material, asdefined above.

Suitably the carbon fibre precursor of this first aspect consists of afibre of polymeric material and a coating layer on the fibre, thecoating layer consisting of a dielectric heating susceptor material, asurfactant and a polymeric carrier material, as defined above.

According to a second aspect of the present invention, there is provideda method of preparing a carbon fibre precursor for a carbon fibreformation process, the method comprising the steps of:

a) providing a fibre of polymeric material;

b) coating the fibre of polymeric material with a composition comprisinga dielectric heating susceptor material.

Suitably the steps of the method of this first aspect are carried in theorder step a) followed by step b).

The fibre of polymeric material, dielectric heating susceptor materialand carbon fibre precursor produced by the method may have any of thesuitable features and advantages described in relation to the firstaspect.

Suitably step b) involves the steps of:

i) contacting the fibre of polymeric material with a liquid comprising apolymeric carrier material;

ii) contacting the fibre of polymeric material with a liquid comprisingthe dielectric heating susceptor material.

Suitably step i) coats the fibre of polymeric material with thepolymeric material and step ii) coats the polymeric carrier materialwith the dielectric heating susceptor material, to form a coating on thefibre of polymeric material comprising both the polymeric carriermaterial and the dielectric heating susceptor material.

Suitably step b) involves dipping the fibre of polymeric material into aliquid comprising the dielectric heating susceptor material.

Suitably step b) involves the steps of:

i) dipping the fibre of polymeric material into the liquid comprisingthe polymeric carrier material;

ii) dipping the fibre of polymeric material into the liquid comprisingthe dielectric heating susceptor material.

The polymeric carrier material may have any of the suitable features andadvantages described in relation to the first aspect.

Suitably the liquid of step i) comprises the polymeric carrier materialin an amount suitable for applying a coating of the desired thicknessonto the fibre of polymeric material. This may depend on the method ofapplication of the liquid to the fibre. Suitably the liquid of step i)comprises 0.1 to 1.0 wt % polymeric carrier material, suitably 0.1 to0.5 wt % polymeric carrier material, suitably wherein the liquid isapplied to the fibre by dipping, suitably wherein the polymeric carriermaterial is an ionic polymer. Suitably the liquid of step i) is anaqueous liquid.

Suitably the liquid of step ii) comprises the dielectric heatingsusceptor material in an amount suitable for applying the desired amountof dielectric heating susceptor material onto the fibre of polymericmaterial. This may depend on the method of application of the liquid tothe fibre. Suitably the liquid of step i) comprises 0.001 to 0.1 wt %dielectric heating susceptor material, suitably 0.01 to 0.1 wt %dielectric heating susceptor material, suitably 0.03 to 0.07 wt %suitably wherein the liquid is applied to the fibre by dipping, suitablywherein the dielectric heating susceptor material is MWCNTs. Suitablythe liquid of step ii) is an aqueous liquid.

Suitably the steps i) and ii) are repeated at least once.

Steps i) and ii) may be carried out in the order step i) followed bystep ii), or in the reverse order. Steps i) and ii) may each be carriedout multiple times, either sequentially or alternately. Suitably stepsi) and ii) are carried out alternately and are repeated multiple times.Suitably steps i) and ii) are both repeated from 3 to 15 times, suitablyfrom 5 to 12 times, suitably from 5 to 10 times. The inventors havefound that repeating the steps i) and ii) in this way may provide aneffective coating comprising dielectric heating susceptor materialwithout adversely affecting the carbon fibre precursors, for example byinducing undesirable agglomeration of the carbon fibre precursors.

Suitably after step i) and before step ii) the fibre of polymericmaterial is rinsed with a solvent. Suitably the solvent is water.Suitably after step ii) and before a repeat of step i) the fibre ofpolymeric material is rinsed with a solvent. Suitably the solvent iswater.

Suitably after step i) and before step ii) the fibre of polymericmaterial is dried, suitably after being rinsed with a solvent such aswater. Suitably after step ii) and before a repeat of step i) the fibreof polymeric material is dried, suitably after being rinsed with asolvent such as water.

Suitably the liquid comprising the dielectric heating susceptor materialfurther comprises a surfactant. The surfactant may have any of thesuitable features and advantages described in relation to the firstaspect. Suitably the surfactant is present in an amount which providesfrom 0.1 to 5.0 wt % of the liquid comprising the dielectric heatingsusceptor material, suitably from 0.1 to 2.0 wt %, suitably from 0.5 to1.5 wt %, suitably wherein the surfactant is an ionic surfactant.

The method of this second aspect suitably provides a carbon fibreprecursor according to the first aspect which is suitable forcarbonizing to form a carbon fibre using dielectric heating andtherefore may be carbonized in a more energy efficient process thanknown carbon fibre precursors which require conventional oven or furnaceheating.

According to a third aspect of the present invention, there is provideda method of forming a carbon fibre, the method comprising the steps of:

1) preparing a carbon fibre precursor according to a method of thesecond aspect;

2) exposing the carbon fibre precursor to electromagnetic radiation toheat the carbon fibre precursor to a temperature of at least 800° C. tocarbonize the carbon fibre precursor to form the carbon fibre.

The steps of the method are carried out in the order step 1) followed bystep 2).

Suitably in step 2) the carbon fibre precursor is heated to atemperature of from 800° C. to 2,000° C., suitably from 800° C. to1,500° C., suitably from 900° C. to 1,200° C.

Suitably the electromagnetic radiation is microwave frequency radiationor radio frequency radiation. Suitably the electromagnetic radiation ismicrowave frequency radiation, suitably having a frequency of from 1 to300 GHz. Suitably step 2) is carried out in a microwave heater, forexample a microwave oven, for example having a frequency of microwaveradiation of 2.45 GHz and a power output of 700 W. Suitably step 2) iscarried using a microwave heater having a power output in the range 100to 700 W.

Suitably step 2) involves exposing the carbon fibre precursor tomicrowave frequency radiation for 2 to 60 minutes, suitably 5 to 45minutes, suitably 10 to 30 minutes, for example approximately 20minutes.

Suitably step 2) involves exposing the carbon fibre precursor tomicrowave frequency radiation of frequency 1 to 300 GHz, of power outputof 700 W for 2 to 60 minutes, suitably 5 to 45 minutes, suitably 10 to30 minutes, for example approximately 20 minutes.

According to the present invention, there is provided a carbon fibreprecursor and/or a method as described above, and/or with reference tothe accompanying drawings.

EXAMPLES

Materials

Fibres of a 60/40 blend of lignin (TCC)/TPU were produced from modified(hydroxy propyl) Kraft hardwood (TCC) with a Mw of 11,357 g/mol obtainedfrom Tecnaro co. (Germany) and the TPU (thermoplastic polyurethane)Pearlthane ECO 12T95 obtained from Veltox (France) (manufactured byLubrizol). The blended 60/40 lignin (TCC)/TPU was extruded using anXplore microcompounder MC15 twice, the first time to form pellets andthe second time to form fibres to provide the fibres of polymericmaterial of the carbon fibre precursors of the present invention. In thefirst extrusion, the pellets were extruded in a counter-rotating modeusing a separated heating control at temperatures of 175, 190, 200 and190° C. to provide pellets with a diameter and/or length ofapproximately 5 mm. To form the fibres, the pellets were extruded usinga single hole die with a diameter of 500 microns at temperatures of 155,190, 200 and 190° C. This provided fibres having a diameter of from 100to 200 μm. As they were formed, the fibres were wound onto bobbins usingan automatic winder to provide fibres.

Spun PAN fibres were obtained from Dralon.

Multi-walled carbon nanotubes (MWCNTs) “Elicarb” were obtained fromThomas Swan and Co. Ltd.

Poly(diallyldimethylammonium chloride) (PDDA), with a molecular weightof 100,000-200,000 g/mol and sodium deoxycholate (DOC) (C₂₄H₃₉NaO₄) werepurchased from Sigma Aldrich.

Sample Preparation

Example 1

The 60/40 blend of lignin (TCC)/TPU polymer fibres were coated accordingto the following layer-by-layer process which is also summarized inschematic 100 of FIG. 1. 0.05 wt % of MWCNTs were dispersed in anaqueous solution comprising 1 wt % DOC to provide suspension 102. TheMWCNT/DOC suspension 102 was sonicated for 30 min, followed by 20 min of15 W tip sonication in an ice water bath, and another 30 min of bathsonication to homogenize. The MWCNT/DOC suspension was then centrifugedat 4000 rpm for 20 min and the supernatant was decanted. The polymerfibres were immersed in a cationic PDDA (0.25 wt) solution 101 for 5min, followed by rinsing and drying, and then dipped into the anionicMWCNT/DOC suspension 102 for another 5 min. This process results in thedeposition on the polymer fibres of a PDDA/MWCNT-DOC bilayer (BL). Afterthe initial BL was deposited, all subsequent layers were deposited with2 min dip times, with rinsing and drying in between. This cycle wasrepeated to deposit the desired number of bilayers. Deposited multilayerfilms were air-dried overnight and then stored in a desiccator prior tofurther processing or characterization. The Examples summarized in Table1 are named 1.xy, with the number “x” denoting the number of coatingcycles the sample has undergone. The “y” letter in the Example namedenotes the sequence of steps involved in each coating cycle (shown inthe “procedure” column), with “a” denoting adip-rinse-rinse-dip-rinse-rinse sequence and “b” denoting adip-rinse-dry-dip-rinse-dry sequence.

In order to improve the adhesion of the MWCNTs layers, the surface ofthe fibres can be treated using surface activation techniques such asmicrowave plasma surface modification, dielectric barrier dischargesurface modification or atmospheric pressure jet plasma surfacemodification. These samples were treated using Dielectric BarrierDischarge (DBD) Plasma Remote in a System SURFX Atomflo 500 (13.56 MHzTgas 90-110° C.).

TABLE 1 Lignin-TPU/MWCNTs coated samples. Surface Sample Susceptormodification Cycles Procedure Example 1.5a MWCNTs Yes 5dip-rinse-rinse-dip- rinse-rinse Example 1.10a MWCNTs Yes 10dip-rinse-rinse-dip- rinse-rinse Example 1.5b MWCNTs Yes 5dip-rinse-dry-dip- rinse-dry Example 1.10b MWCNTs Yes 10dip-rinse-dry-dip- rinse-dry Example 1.20b MWCNTs Yes 20dip-rinse-dry-dip- rinse-dry Example 1.30b MWCNTs Yes 30dip-rinse-dry-dip- rinse-dry

Example 2—PAN Coated Samples

PAN polymer fibres were coated with PDDA/MWCNT-DOC bilayers using thesame procedure to provide the carbon fibre precursor of Example 2.During each bilayer coating process, the PAN fibers changed their colourto grey due to the presence of MWCNTs. Table 2 summarizes the PAN-basedcoated carbon fibre precursor examples. These Examples are numbered inthe same way as described above for Example 1.

TABLE 2 Surface Sample Susceptor modification Cycles Procedure Example2.5a MWCNTs Yes 5 dip-rinse-rinse-dip- rinse-rinse Example 2.4a MWCNTsYes 4 dip-rinse-rinse-dip- rinse-rinse Example 2.3b MWCNTs Yes 3dip-rinse-dry-dip- rinse-dry Example 2.2b MWCNTs Yes 2dip-rinse-dry-dip- rinse-dry Example 2.1b MWCNTs Yes 1dip-rinse-dry-dip- rinse-dry

After the coating process, the PAN fibres were not agglomerated—see SEMimages of FIG. 2 which show control and PAN fibres coated with 5 cyclesof the above process—indicating that the fibres were coated individuallyand homogeneously, which was desirable for the carbonization process.

FIG. 3 shows SEM images of Lignin based precursor fibres coated withMWCNTs. The top three images are of Example 1.5a; the middle images areof Example 1.10a; and the bottom images are of Example 1.20b. The SEMimages of FIG. 3 show a very homogeneous coating on the surface of theExample 1 lignin-based carbon fibre precursors. Several regions of thefibre were analysed observing the same degree of coating in all of thezones. Where the fibres were coated with 20 cycles (Example 1.20b), therelatively high amount of MWCNTs produced agglomerates, as can be seenin FIG. 3. Therefore, the optimum number of coating cycles, at least forthese fibres, may be between 5 and 10.

FIG. 4 shows the heating profile of the carbon fibre precursors ofExample 2.5a under microwave heating using a modified domestic microwaveoven with an IR-sensor incorporated for temperature monitoring. Thisheating profile shows it is possible to reach a temperature of above900° C., which is sufficient to carbonize the carbon fibre precursors toproduce carbon fibres.

FIG. 5 shows the heating profiles at maximum microwave power of theExamples of Table 2. In these heating profiles, the temperatureincreases until 1000° C. is reached (1000° C. is the limit of thetemperature detector). The heating profiles show a rapid increase in theinitial phase of heating. However, after a certain amount of time thetemperature decreases slowly to below 1000° C. It is possible to observethat the temperature decreases faster when the samples have been coatedwith a fewer number of cycles. The anomalous behaviour of the sample 4Cmay be attributed to some experimental errors during sample preparation.

FIG. 6 shows heating profiles of Example 2.5a as a function of MWpower—33%, 55% and 77% of the maximum 700 W power for the medium high,medium and medium low levels, respectively (in the order from top tobottom).

The carbon fibre produced from Example 2.5a as described above (PANfibres coated with 5 cycles after 10 minutes of microwave heating) wasanalyzed by Raman in order to assess whether the microwave heatingcarbonized the polymer fibres of the carbon fibre precursorssuccessfully. FIG. 7 shows the Raman spectrum of the carbon fibresproduced from Example 2.5a after 10 minutes of microwave heating. Thisspectra has the typical features observed for carbon fibres—the D bandlocated at 1351 cm⁻¹ and the G band located at 1583 cm⁻¹. In addition,SEM images were taken of these carbon fibres. These images, shown inFIG. 8, show that the fibres kept their fibrous shape duringcarbonization and would therefore be suitable for industrial use.

In summary, the present invention provides carbon fibre precursors foruse in the formation of carbon fibre materials. The carbon fibreprecursors comprise fibres of polymeric material which have a coatinglayer thereon, the coating layer comprising a material susceptible todielectric heating, for example carbon nanotubes. The carbon fibreprecursors may be suitable for forming into carbon fibres using adielectric heating step, despite the fibres of polymeric material notbeing susceptible to dielectric heating, without adversely affecting thestructure and physical properties of the main body of the carbon fibreso formed. A method of preparing a carbon fibre precursor for a carbonfibre formation process and a method forming a carbon fibre are alsoprovided.

Although a few preferred embodiments have been shown and described, itwill be appreciated by those skilled in the art that various changes andmodifications might be made without departing from the scope of theinvention, as defined in the appended claims.

Throughout this specification, the term “comprising” or “comprises”means including the component(s) specified but not to the exclusion ofthe presence of other components. The term “consisting essentially of”or “consists essentially of” means including the components specifiedbut excluding other components except for materials present asimpurities, unavoidable materials present as a result of processes usedto provide the components, and components added for a purpose other thanachieving the technical effect of the invention. Typically, whenreferring to compositions, a composition consisting essentially of a setof components will comprise less than 5% by weight, typically less than3% by weight, more typically less than 1% by weight of non-specifiedcomponents.

The term “consisting of” or “consists of” means including the componentsspecified but excluding addition of other components.

Whenever appropriate, depending upon the context, the use of the term“comprises” or “comprising” may also be taken to encompass or includethe meaning “consists essentially of” or “consisting essentially of”,and may also be taken to include the meaning “consists of” or“consisting of”.

For the avoidance of doubt, wherein amounts of components in acomposition are described in wt %, this means the weight percentage ofthe specified component in relation to the whole composition referredto. For example, “wherein the liquid comprises 0.1 to 1.0 wt % polymericcarrier material” means that from 0.1 to 1.0 wt % of the liquid isprovided by the polymeric carrier material.

The optional features set out herein may be used either individually orin combination with each other where appropriate and particularly in thecombinations as set out in the accompanying claims. The optionalfeatures for each aspect or exemplary embodiment of the invention as setout herein are also to be read as applicable to any other aspect orexemplary embodiments of the invention, where appropriate. In otherwords, the skilled person reading this specification should consider theoptional features for each exemplary embodiment of the invention asinterchangeable and combinable between different exemplary embodiments.

Attention is directed to all papers and documents which are filedconcurrently with or previous to this specification in connection withthis application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, and drawings), and/or all of the steps of anymethod or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, and drawings), or to any novel one, or anynovel combination, of the steps of any method or process so disclosed.

1. A carbon fibre precursor comprising a fibre of polymeric material anda coating layer on the fibre, the coating layer comprising a dielectricheating susceptor material.
 2. The carbon fibre precursor according toclaim 1, wherein the coating layer has a thickness of from 5 to 200 nm.3. The carbon fibre precursor according to claim 1, wherein coatinglayer comprises a surfactant.
 4. The carbon fibre precursor according toclaim 1, wherein the coating later comprises a polymeric carriermaterial.
 5. The carbon fibre precursor according to claim 1, whereinthe fibre of polymeric material comprises lignin.
 6. The carbon fibreprecursor according to claim 1, wherein the dielectric heating susceptormaterial is formed of carbon nanotubes.
 7. The carbon fibre precursoraccording to claim 1, wherein the dielectric heating susceptor materialprovides from 0.01 to 0.1 wt % of the carbon fibre precursor.
 8. Amethod of preparing a carbon fibre precursor for a carbon fibreformation process, the method comprising the steps of: a) providing afibre of polymeric material; b) coating the fibre of polymeric materialwith a composition comprising a dielectric heating susceptor material.9. The method according to claim 8, wherein step b) involves dipping thefibre of polymeric material into a liquid comprising the dielectricheating susceptor material.
 10. The method according to claim 9, whereinstep b) involves the steps of: i) dipping the fibre of polymericmaterial into a liquid comprising a polymeric carrier material; ii)after step i) dipping the fibre of polymeric material into the liquidcomprising the dielectric heating susceptor material.
 11. The methodaccording to claim 10, wherein after step i) and before step ii) thefibre of polymeric material is rinsed with a solvent.
 12. The methodaccording to claim 10, wherein the steps i) and ii) are repeated atleast once.
 13. The method according to claim 9, wherein the liquidcomprising the dielectric heating susceptor material further comprises asurfactant.
 14. The method according to claim 8, further comprising: c)exposing the carbon fibre precursor to electromagnetic radiation to heatthe carbon fibre precursor to a temperature of at least 800° C. tocarbonize the carbon fibre precursor to form the carbon fibre.
 15. Themethod according to claim 14, wherein step c) involves exposing thecarbon fibre precursor to microwave frequency radiation having afrequency of from 1 to 300 GHz for 2 to 60 minutes.